1. Field of the Invention
The present invention relates to a porous film having excellent flexibility, moisture permeability and stretchability, as well as to a process for production thereof. More particularly, the present invention relates to a porous film which is made of a composition comprising a particular ethylene-.alpha.-olefin copolymer, a particular thermoplastic elastomer and an inorganic filler and which has excellent flexibility, moisture permeability and stretchability, as well as to a process for production thereof.
2. Description of the Prior Art
The applications of porous films include a back sheet of disposable diaper. The porous film used in such an application is required to have the following properties. Firstly, the porous film must have good moisture permeability in order to prevent damp when a disposable diaper using said film is worn. Secondly, the porous film is required to have good touch and be flexible so as not to prevent the movement of a person wearing the Paper diaper. Further, the porous film is required to have a small residual strain after elongation, i.e. good stretchability, because a porous film having a large residual strain does not regain its original size after elongation, gives no feeling of fitness, and slip down.
Various methods have been proposed in order to obtain a polyolefin porous film having flexibility. The methods are largely divided into two methods, i.e. a method A which comprises adding a rubber-like component and a waxy or liquid component to a polyolefin resin and a method B which comprises employing a low modulus of elasticity resin as the matrix resin.
Typical examples of the method A include the followings. U.S. Pat. No. 4,472,328 discloses a method for obtaining a moisture-permeable film having flexibility by orientation of a film or sheet made from a composition comprising a linear low-density polyethylene (LLDPE) resin, a filler and a waxy or liquid hydrocarbon polymer. This moisture-permeable film or sheet, however, has no flexibility and stretchability as intended by the present invention. Japanese Patent Application Kokai (Laid-Open) No. 295649/1988 discloses a method for obtaining a film of high strength and low unevenness of stretching by adding, to a linear low-density polyethylene, an olefin terpolymer such as ethylene-propylene-diene copolymer or the like. The film has flexibility and moisture permeability owing to the addition of an olefin terpolymer, but has no sufficient stretchability. Increase in the addition amount of the olefin terpolymer in order to obtain higher stretchability results in a film of low moisture permeability.
The method B includes the followings. U.S. Pat. No. 4,698,372 discloses the use, as the matrix polymer, of an orientatable thermoplastic polymer having a low modulus of elasticity and a hydrophobic surface. An ethylene/propylene/1,4-hexadiene terpolymer is shown as an example of the matrix polymer. The matrix polymer, however, has a low melting point as appreciated from the fact that stretching at normal temperature is conducted. The resulting film, therefore, has high thermal shrinkage and its moisture permeability changes with a small heat. Thus, the film has no practical applicability. Japanese Patent Publication No. 82010/1992 discloses the use of an ethylene-.alpha.-olefin copolymer having a density of 0.86-0.91 g/cm.sup.3, a boiling n-hexane insoluble of 10% by weight or more and a DSC peak temperature of 100.degree. C. or more. The gas-permeable film using such a resin having the above properties has flexibility presumably owing to the presence of low molecular components but has no stretchability. Japanese Patent Application Kokai (Laid-Open) No. 50522/1993 discloses the use of an ethylene-propylene-diene copolymer/an ethylene-vinyl acetate copolymer. The films shown in Examples have flexibility and stretchability but are not satisfactory in moisture permeability. Moreover, they tend to emit an odor or cause blocking. Japanese Patent Publication No. 18856/1993 proposes a porous sheet having flexibility and stretchability, obtained by adding an inorganic filler to a polyolefin elastomer or a resin composition between said polyolefin elastomer and a polyolefin and stretching the resulting composition. As the polyolefin elastomer, there is used a resin composition comprising, as the main component, a soft resin such as ethylene-propylene copolymer, ethylene-butene copolymer or the like. The porous sheet using such a soft resin as the matrix, however, has problems, for example, no sufficient stretchability. Japanese Patent Application Kokai (Laid-Open) No. 204936/1989 discloses a moisture-permeable film comprising a linear ultralow-density polyethylene having a density smaller than 0.910 g/cm.sup.3, a butadiene rubber and a filler. This moisture-permeable film, however, has a yield point as mentioned in the Examples of the document. A moisture-permeable film having a yield point, when elongated by a tension larger than the yield point and then subjected to tension relaxation, does not return to the original state, that is, has a large residual strain after the elongation.
In view of the above problems of the prior art, the present invention has an object of producing a porous film having flexibility, excellent moisture permeability and good stretchability.